Hitherto, there are known radiation imaging apparatuses which image an inner structure of an object by transmitting a radiation through the object. In a general radiation imaging apparatus among these apparatuses, a radiation is radiated to an object to be transmitted through the object and a projected image of the radiation is imaged. In such a projected image, shading appears depending on the ease of the transmission of the radiation and this shading indicates an inner structure of the object.
In such a radiation imaging apparatus, only an object having a property of absorbing a radiation to some extent can be imaged. For example, a soft tissue of a living body hardly absorbs a radiation. Even when such a tissue is imaged by a general apparatus, almost nothing appears on the projected image. In this way, there is a limit in principle of the general radiation imaging apparatus at the time of imaging the inner structure of the object which hardly absorbs the radiation.
Here, a radiation phase difference imaging apparatus which images the inner structure of the object by using the phase difference of the transmitted radiation can be considered. Such an apparatus images the inner structure of the object by using Talbot interference.
The Talbot interference will be described. A radiation having the same phase is radiated from a radiation source 53 of FIG. 10. When the radiation passes through a phase grating 55 having a fabric shape, an image of the phase grating 55 appears on a projection surface separated from the phase grating 55 by a predetermined distance (Talbot distance). This image is called a self-image. The self-image is not a simple projected image of the phase grating 55. The self-image is formed only at a position in which the projection surface is separated from the phase grating 55 by the Talbot distance. The self-image is formed by interference fringes caused by light interference. The self-image of the phase grating 55 appears at the Talbot distance because the phases of the radiation generated from the radiation source 53 are aligned. When the phases of the radiation are disturbed, the self-image appearing at the Talbot distance is also disturbed.
The radiation phase difference imaging apparatus images the inner structure of the object by using the disturbance of the self-image. An object is placed between the radiation source and the phase grating 55. Since the object hardly absorbs the radiation, most of the radiation incident to the object is radiated toward the phase grating 55.
The radiation does not completely pass through the object. The phase of the radiation changes while the radiation passes through the object. The radiation radiated from the object passes through the phase grating 55 while the phase is changed. When the radiation is observed on the projection surface positioned at the Talbot distance, the self-image of the phase grating 55 is disturbed. The disturbance degree of the self-image indicates a change in phase of the radiation.
The degree to which the phase of the radiation transmitted through the object specifically changes depends on a position in which the radiation passes through the object. If the object is homogeneous, a change in phase of the radiation might be the same throughout the object. However, the object generally has a certain inner structure. When the radiation is transmitted through such an object, a change in phase is not uniform.
Thus, the inner structure of the object can be understood when a change in phase can be understood. A change in phase can be understood by observing the self-image of the phase grating 55 at the Talbot distance.
The self-image of the phase grating 55 is detected by a radiation detector which is separated from the phase grating 55 by a predetermined distance. The separation distance between the phase grating 55 and the radiation detector may not be set arbitrarily. When the separation distance is not suitable, the self-image is not captured on the radiation detector. The suitable separation distance is determined by the distance from the radiation source 53 to the phase grating 55, the finesse of the fabric shape forming the phase grating 55, and the wavelength of the radiation output from the radiation source 53. Patent Document 1 introduces mathematical expressions relating to these parameters. When the suitable separation distance needs to be obtained, other parameters such as a distance from the radiation source 53 to the phase grating 55 may be applied to a mathematical expression.